The present invention relates to a method of sequentially transporting a first and a second fluid through a conduit.
Sequential transporting of two fluids through a conduit is frequently encountered in the process industry, e.g. around refineries, petrochemical and chemical plants. Multi-product pipeline conduits are used to transport fluids over short distances, such as on a plant site between one unit or tank and another, but also over long distances, for tens, hundreds or more kilometers. The fluids can be liquids, in the case of a refinery for example mogas, gasoil, kerosene or other refinery streams of different product qualities and grades. The fluids can also be gases such as natural gas and nitrogen, for example in cases where nitrogen is occasionally used for purging a natural gas pipeline.
A problem that is commonly encountered when transporting different fluids sequentially through a pipeline is mixing between the tail end of the first fluid batch and the front end of the second fluid. If the transport takes place over long distances, the length of pipeline containing both first and second fluid can be substantial, several hundreds or even thousands of meters long. Generally one obtains large amounts of off-spec material in this way, which has to be downgraded or is even wasted.
Several effects contribute to this. Mixing of fluids at a microscopic level takes place due to diffusion, turbulent dispersion and residence time distribution effects at the interface. Macroscopically, a particularly serious problem is encountered in non-vertical, in particular (nearly) horizontal conduits, when the fluids happen to stratify in the conduit, due to a density difference between the fluids. For example, when the second fluid that is fed into the pipeline after the first fluid, is lighter than the first fluid, the second fluid may not fully displace the first fluid from the full cross-section of the pipeline but may float over the first fluid so that two moving layers of fluid are obtained. A heavier second fluid can also shift under a lighter first fluid. It is particularly in such a case of stratification that very long lengths of pipeline are filled with both fluids. In many cases only limited actual microscopic mixing between the fluids at the nearly horizontal interface is observed.
This problem is currently dealt with in various ways. If the pipeline is short, one may simply accept the situation and dispose of the off-spec material obtained. A common way to prevent mixing and stratification is to use a mechanical separation by separating plugs, such as spheres from a flexible material, often referred to as “pigs”. A problem associated with these plugs is that they can get stuck along the pipeline, and that they may also cause unsafe situations particularly at the end of the pipeline where they have to be separated from the fluids. Also, a launch facility for the plugs at the pipeline inlet is needed. An alternative for a flexible sphere is to use a plug of gel (“gel pigs”). Contrary to fluids, a gel exhibits a finite yield stress which is the stress at which the gel begins to flow. Below the yield stress, the gel behaves essentially as a flexible solid, and it is in this regime that gel pigs are operated. Gel pigs are however generally rather expensive since relatively large amounts of chemicals are required to fill a sufficiently long length of pipeline. Gel pigs also introduce substances different from the process fluids to be transported into the pipeline, and are therefore a cause of contamination.
International Patent Application Publication No. WO 95/12741 discloses a method for displacing fluid from and cleaning a wellbore space of a vertical subterranean well. In the known method a sequence of different fluids is circulated into the well, first a displacement fluid, then a drive fluid, followed by a buffer fluid and a wash fluid. A viscous gel solution is used as drive fluid, to obtain “piston-like” characteristic for displacing displacement fluid without substantial mixing. Chloride brine or seawater buffer fluid is used between the drive and the wash fluid because it is expected that mixing with the drive fluid occurs, and can be provided in a density so as to mitigate large density differentials.